Io's volcanic activity has been monitored by instruments
aboard the Galileo spacecraft since June 28, 1996. We
present results from observations by the Near-Infrared
Mapping Spectrometer (NIMS) for Galileo and the first 3
orbits of the Galileo Europa Mission. We correlate these
results with those from Galileo's Solid State System (SSI),
from ground-based observations, and from observations made
during the two Voyager fly-bys in 1979. A total of 61 active
volcanic centers (hot spots and plumes sites) were
identified from Galileo, Voyager, and ground-based
observations. Of these, 48 are hot spots detected by NIMS
and/or SSI. The location of plumes shows a strong equatorial
preference, however, the distribution of active volcanic
centers does not show any clear correlation with latitude,
longitude, Voyager-derived topography, or with heat flow
patterns predicted by competing tidal dissipation models.
Hot spots are correlated with surface colors, particularly
dark and red deposits, and generally anti-correlated with
white, SO2-rich areas. Volcanic features on the surface,
such as calderas and flows, were identified in the Galileo
and Voyager images at all of the hot spot locations. Hot
spot temperatures obtained from both NIMS and SSI are
consistent with silicate volcanism, which appears to be
widespread on Io. The activity of hot spots has been
monitored by Galileo since 1996 and results indicate that
two types of activity are present: persistent-type, lasting
from months to years, and sporadic events, which may
represent intermittent activity or activity that often falls
to levels below the detection limits of the instruments.
Sporadic events are not often detected, but may make an
important contribution to Io's heat flow and resurfacing.